Perhaps because half staff, being used with polearms, still allows you some reach, where as half-sword doesn't, due to the length of the weapon and due to how they are held. Also one can enter or exit a half-sword at will.

I'd also argue that moving from half-staff to quarter staff is very easy with a pole-arm whereas a sword, particularly with it being shorter and having to work with its edges, makes it a bit more work, plus they aren't exactly balanced for constantly moving it in that way. A true great sword might well fall into the pole-arm category in this case, though.